1. Field of the Invention
The present invention relates to a block copolymer preferably used as a polymer electrolyte, in particular, as a member for a fuel cell.
2. Description of the Related Art
A polymer having proton conductivity, namely a polymer electrolyte is used for materials constituting a barrier membrane of electrochemical devices such as a primary cell, a secondary cell and a solid polymer fuel cell. For example, polymer electrolytes in which a polymer having perfluoroalkylsulfonic acid as a super strong acid in the side chains and a perfluoroalkane chain in the main chain is an active component, such as NAFION (Registered TradeMark of EI DuPont de Nemours & Company), have been conventionally used mainly because of the superior electric generation property when they are used as a barrier membrane material for a fuel cell. However, problems have been indicated that this kind of material is very expensive, heat resistance is low, membrane strength is low and it cannot be practically used without any reinforcement, etc.
Under these circumstances, the development of a polymer electrolyte with low price and superior in performance that can replace the above-mentioned polymer electrolytes has been recently activated.
For example, there is proposed a block copolymer comprising a segment having no sulfonic acid group and a segment having a sulfonic acid group, in which the former segment is polyethersulfone and the latter segment has the ether bound moieties of diphenylsulfone with bisphenol having a sulfonic acid group as a repeating unit (for example, refer to Japanese Unexamined Patent Publication No.2003-031232). However, the further improvement of proton conductivity. (ion conductivity) is desired for practical use of fuel cells and when the total number of ion exchange groups is increase for merely improving the ion conductivity, water resistance tends to be deteriorated.